A current source is an electrical device that maintains a constant current regardless of changes in voltage across it. Current sources can be constructed in various ways. FIG. 1 shows one approach known in the art as a current-regulating diode (or “CR diode”) 100. The CR diode 100 is typically constructed from an n-channel junction field effect transistor (JFET) but may also be constructed from an n-channel depletion mode metal-oxide-semiconductor FET (MOSFET) 102, as in FIG. 1. Both the n-channel JFET and n-channel depletion mode MOSFET are ‘normally ON,’ meaning that they have a negative threshold voltage Vth and can only be turned OFF if their gate-source voltage VGS<Vth. In the CR diode 100 depicted in FIG. 1 the gate-source voltage is VGS=0. Accordingly, so long as the drain voltage VD remains greater than −Vth, the FET 102 remains ON and the CR diode 100 maintains (i.e., regulates) a constant current IDSS proportional to the square of the FET's 102's threshold voltage Vth, i.e., IDSS ∝(Vth)2.
The CR diode 100 is only capable of regulating a single fixed current IDSS. If a different and lower current is needed, a variable resistor RCR 204 can be inserted between the source of the FET 202 and ground, as illustrated in FIG. 2. So long as VGS>Vth and VDS remains higher than the knee voltage VK, i.e., VDS>VK=VGS−Vth, the CR diode 200 is able to regulate various values of drain current ID1, ID2, . . . , etc., each having a value depending on the resistance setting of the variable resistor RCR 204.
Although the adjustable-current CR diode 200 is able to regulate different values of drain current ID1, ID2, . . . , etc., it is not an optimal solution, especially when high precision is required. One problem relates to the fact that CR diodes are most often used in integrated circuits but there is no way to adjust the variable resistor RCR 204 when implemented in an integrated circuit. Another problem is that even in situations where the CR diode 200 is designed to have a fixed resistor to realize a specific desired ID and is manufactured in an IC, the resulting CR diode 200 does not always produce the current ID according to the design. Deviation from the intended (i.e., designed) current ID is caused by inconsistencies and variabilities in the IC manufacturing processes, which not only can affect the ability to manufacture high precision resistors repeatedly but which also have an impact on the intended performance of the CR diode's 200's FET 202. Yet another problem with prior art CR diodes is that their output conductance gOUT=∂DS/∂VDS can be higher than desired in some applications. The output conductance gOUT provides an indication of how effective the CR diode is at serving as a current source, in other words, how effective it is at regulating its current for a given value of VGS. A CR diode having a high output conductance gOUT is problematic since it means that the CR diode is sensitive to changes in the voltage across it and, as a result, is unable to regulate its current to the extent necessary or desired.